From "Tips and Notes" from "Letters 3":
Ukrainian verb infinitives (initial forms of verbs you find in dictionaries) always have ending –ти. For example: пити (to drink); я люблю пити сік (I like to drink juice).
The tips/notes say "я їм" so why are we not saying "я їмти" for the infinitive? Confused.
I'm pretty sure that їм is just the я form of the verb, and that їсти is the infinitive. So, similar to Spanish where when you say "I want to read", it would be (at least I think) "Quiero leer." while "I read" is "Leo."? Although you may understand that already--perhaps їсти is somewhat irregular? I'm not actually sure.
Eta: https://en.m.wikipedia.org/wiki/Ukrainian_grammar#Athematic_verbs looks like їсти is a little unusual in present tense as verbs go.